This application claims the priority of Korean Patent Application No. 2003-79604, filed on Nov. 11, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an optical amplifier used for optical communication, specifically, to a long-wavelength band (L-band) optical amplifier (where an L-band ranges from 1570 nm to 1610 nm) capable of maintaining a constant gain by using a backward amplified spontaneous emission, thereby providing a stable optical output power independent of a power or a wavelength of an input signal.
2. Description of the Related Art
An L-band optical amplifier is expected to be widely used in a wavelength division multiplexing (WDM) transmission system as well as in a metro WDM transmission system. In an L-band optical amplifier used in such transmission systems, it should be transient-free under abrupt changes in input power or number of channels.
However, the gain of an L-band optical amplifier is reported to be different depending on the wavelength of the input signal, although the power of the signal is the same. This gain difference comes from the low gain coefficient in the L-band, significantly long erbium-doped fiber (EDF) and consequent strong backward amplified spontaneous emission (ASE), which saturates the gain in the front section of an L-band optical amplifier. The gain difference of an L-band optical amplifier may cause an optical output to change greatly.
Pump-controlled optical amplifiers employing feed-forward schemes are widely used in the C-band (1530 nm˜1565 nm). Typically, the total input power, irrespective of signal wavelength, is detected in front of the optical amplifier and then the pump power is adjusted to the corresponding preset value. This method is simple to incorporate and fast in control, compared to a feed-back scheme in which the intrinsically slower process can induce the higher gain transients in the surviving channels. However, since a conventional pump-controlled L-band optical amplifier uses the same pump power for the same input power independent of a wavelength of an input signal, it is difficult to control a change in an output power caused by a gain difference depending on the wavelength of the input signal. Consequently, transients from conventional L-band optical amplifiers may be great.